The precautionary principle basically says if an action or policy might cause harm to the environment, then even if there is no proof the action will cause harm the burden of proof is on those advocating the action to prove the action would not be harmful.

BAWG, incidentally, defines itself as:

A diverse collaborative of individuals and organizations who are dedicated to protecting health and the environment. We recognize that fundamental changes in decision-making need to happen in order to build healthier, more just, and sustainable communities.

The attendant article on Wikipedia states:

This principle allows policy makers to make discretionary decisions in situations where there is evidence of potential harm in the absence of complete scientific proof. The principle implies that there is a social responsibility to protect the public from exposure to harm, when scientific investigation has found a plausible risk. These protections can be relaxed only if further scientific findings emerge that provide sound evidence that no harm will result.

In the European Union, the application of this principle has been made a statutory requirement.

There are a few problems with this approach where environmental policy is concerned. First, it is impossible to prove a negative. You cannot, for example, prove that something does not exist, only that it does or that it has not been observed. Second, and perhaps more importantly, the principle is generally used to deny an action, but rarely is it used to examine the converse. That is, would more harm be caused by not taking the proposed action than would be caused by taking it? The biggest issue, of course, is that the principle is more often than not used for ideological purposes rather than actual scientific ones.